Use inline to vastly simplify the exposed API. This allows a
comptime-known endian parameter to be propogated, making extra functions
for a specific endianness completely unnecessary.
The idea here is to avoid code bloat by having only one actual io.Reader
implementation, which is type erased, and then implement a GenericReader
that preserves type information on top of that as thin glue code.
The strategy here is for that glue code to `@errSetCast` the result of
the type-erased reader functions, however, while trying to do that I
ran into #17343.
